1. Field of the Invention
The present invention relates to a spherical aberration corrector, method of spherical aberration correction, and charged particle beam instrument.
2. Description of Related Art
Spherical aberration correctors of the two stage three-fold-field type, i.e., using thick fields having three-fold symmetry (that is, using three-fold symmetric fields having a thickness along the optical axis), are used as spherical aberration correctors in electron microscopes such as transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM).
It is known that, if two-fold astigmatism is induced at an optically different plane, a deviation of the circularity of an image (deviation in aspect ratio) or a deviation of the circularity of a diffraction pattern occurs (see, for example, O. Scherzer, Optik 2 (1947), 114).
The spherical aberration corrector of the two stage three-fold-field type uses an optical system employing a thick field of three-fold symmetry and, therefore, if an axial misalignment occurs within a multipolar field, a field of two-fold symmetry is generated in a plane different from a reciprocal space that forms the center of the multipolar field due to an axial misalignment of three-fold astigmatism. This may cause a deviation of the circularity of the image or diffraction pattern.
A spherical aberration corrector of the two stage three-fold-field type is generally fitted with a deflection system for deflecting the electron beam in two dimensions for alignment. It is possible to correct the deviation of the circularity of the image or diffraction pattern using the deflection system.
On the other hand, a spherical aberration corrector needs to correct on-axis aberrations including two-fold astigmatism, on-axis comatic aberration, star aberration, and four-fold astigmatism.
For example, JP-A-2013-30278 discloses a spherical aberration corrector having two multipolar coils for producing a hexapole field and two axisymmetric lenses (transfer lenses) interposed between the coils. This known spherical aberration corrector corrects spherical aberration Cs in an objective lens but it is stated that parasitic aberrations (such as first-order astigmatism of two-fold symmetry, second-order comatic aberration of one-fold symmetry, second-order astigmatism of three-fold symmetry, third-order star aberration of two-fold symmetry, and third-order astigmatism of four-fold symmetry) are produced due to positional deviations of polar elements constituting multipolar lenses and magnetic characteristic variations of the material of the polar elements.
In the spherical aberration corrector, on-axis aberrations are generally corrected by the above-described deflection system.
In this way, the spherical aberration corrector can carry out a correction of deviation of the circularity of an image or diffraction pattern and a correction of on-axis aberrations by means of the above-described deflection system. However, in the spherical aberration corrector, a correction of deviation of the circularity of an image or diffraction pattern and a correction of on-axis aberrations must be made using the same deflection system.
Accordingly, the deflection system must be so adjusted that both of deviation of the circularity of an image or diffraction pattern and on-axis aberrations are corrected. This may complicate the adjustment. Furthermore, such an adjustment for correcting both of the deviation of the circularity and the on-axis aberrations may not be possible to achieve. Consequently, the effects of deviation of the circularity of an image or diffraction pattern and the effects of on-axis aberrations may be not reduced.
Therefore, there is a demand for a spherical aberration corrector permitting a correction of deviation of the circularity of an image or diffraction pattern and a correction of on-axis aberrations to be carried out independently.